1. Field of the Invention
This invention pertains to a radio set for an FDM-TDM-radio transmission system.
2. Description of the Prior Art
Radio sets for an FDM-TDM radio transmission system wherein the communication transmission is performed by a pair of radio channels with time division multiplexing are known in the prior art. Between transmission and reception periods in duplex operation, rapid switching between the two is required. Therefore, the transmitter must generate no residual signals as this would cause interference by simulating a reception signal. However, a complete switching off of the transmission frequency signal is not possible since high frequency accuracy upon a subsequent switching on would be degraded.
Radio sets with multiple superheterodyne receivers, i.e., receivers with multiple frequency conversion, are known in the prior art. For example, EP-B1-0 089 649 discloses a radio set with a single superheterodyne receiver wherein the high frequency carrier oscillation of the transmission local oscillator is disposed between the receiving modulation carrier frequencies of two adjacent radio channels and is fed to a mixer. In view of the double use of the modulation carrier generated by the local oscillator, not only an oscillator may be omitted in the receiver, but additionally, interference oscillations can be prevented which otherwise would occur in a device with a plurality of oscillators.
However, such a circuit concept is disadvantageous in that the intermediate frequency signal contains, in addition to the modulation of the received signal, modulation signal components emitted from the transmitter.
In order to playback the received signal (for example, a voice signal without any interference), in particular without any cross-talk, the modulation signal components which are emitted by the transmission device of the radio must be substantially removed or suppressed in the receiving device. This was achieved by two different methods. The first method subtracted the input signal fed to the transmission modulator from the output signal of the demodulator in the low frequency baseband. The second method used a modulation of the receiving mixer (conversion from the first to the second intermediary frequency position) with a counter phase signal for modulating the local oscillator.
Combinations of the two aforementioned methods are known and are explained in detail in DE-AS 1 002 805, for example. Two embodiments of a device are described in the subject matter of DE-AS 1 002 805 for compensating for the modulation components caused by the modulation signal, which is disposed between the transmission and receiving device and is provided with one amplifier and phase shifter each. In the above-identified first embodiment, a pure modulation frequency effective compensation is achieved. In the above-identified second embodiment, the compensation is obtained by multiple or additional modulation.
In order to accommodate the public's desire for mobile telephonic communication, mobile radio service have been further developed during the past few years, which may be roughly defined by the applications of radio telephone service--radio calling service and other mobile radio services such as data radio and service radio. Also by using cordless telephones, the mobility of the user can be expanded. Thereby, the communication transmission between a base station and the associated hand-held mobile station of the cordless telephone is performed through a pair of radio channels, whereby with respect to the radio transmission technique with a cordless telephone the same procedure may be followed as used in a radio device. The cordless telephones which are presently used in the Federal Republic of Germany have a transmitting capacity of ten milliwatts or more. The cordless telephones called CTI have forty radio channels (with FDMA, Frequency Division Multiple Access) available in the 900 MHz frequency band for communication transmission. The bandwidth of a radio channel is 25 kHz, whereby the spacing of the two radio channels required for the duplex operation 45 MHz. Newer cordless telephones named CTL+ have a total of 80 radio channels each two by twenty-five kHz in the frequency band of 885-887 MHz for the first transmitting direction and in the frequency band of 930- 932 MHz for the second transmitting direction, so that a total of 4 MHz available frequency bandwidth can be used.
In addition to the aforementioned FDM-radio transmission system, FDM-TDM-radio transmission systems are known wherein the two multiplex methods, frequency multiplexing and time multiplexing, are used for the communication transmission. An example is the future DECT (Digital European Cordless Telephone), wherein, in all likelihood, twelve duplex channels will be available on one carrier by means of time division multiple access, time division duplex (TDMA - TDD). The twelve or thirteen modulation carriers are expected to be in the frequency band of 1.88 to 1.9 GHz, whereby about 150 radio channels may be used with a frequency bandwidth of about 20 MHz for the communication transmission. Embodiments which may be used as transmitting/receiving devices are, for example described in U.S. Pat. No. 4,903,257 or in EP-A-2-0333419.
In the radio device disclosed in EP-A2-0 292 996, a main local oscillator is provided which is connected to the transmission mixer as well as to the first receiving mixer. The conversion of the transmission signal into the transmission frequency position is performed in the transmitting device, while the conversion of the received signal into the intermediary position is performed in the first receiving mixer. Thus, the transmitting and receiving frequency of the two radio channels required for the duplex operation are equal, whereby the separation of the transmission direction is achieved by using time multiplexing. For this purpose, one bit rate converter is provided in the transmission direction as well as in the receiving direction for increasing or reducing the bit rate of the transmitting and receiving signal.
In a transmitting and receiving circuit wherein the transmission and reception frequencies are equal, a rapid switching between transmission and reception is made possible by two embodiments of a switch circuit which are disclosed in the radio device of EP-A2 0 292 996. In the first embodiment, a reversing switch is disposed between the transmission and reception amplifier, while in the second embodiment, an additional switch is disposed between the transmission mixer and transmission amplifier. In the embodiment known from EP-A2-0 333 419 relating to a transmission and reception device, the switching off of the transmitter in the receiving operating by turning off the transmission amplifier.
As practical tests of such a concept for the switching over between the transmission and reception operation have shown that residual signals of the transmitter appear in the receiving frequency range thereby simulating and interfering with a receiving signal. A complete turning off of the transmission frequency is not possible since high frequency accuracy upon a subsequent switching on would be degraded.